After the bus trip, the Pranksters lived communally at Kesey's place in La Honda and did a lot of acid tripping together. Whenever a group of Hippies do this, it causes an interesting psychological state, known as the All-One. One of the universal insights people seem to experience when they take LSD is that their consciousness is connected to a greater consciousness (God?) that all other consciousness is also connected to. I tend to call it a consciousness matrix. It reminds me of the Internet, where each person's consciousness is like a computer on the Internet.
I'm a little hard-pressed to think how one would research this All-One state and build a bibliography on the subject, but I think that doing so is important and if you have any good references to it, I hope you will let me know. The best place I've seen it described is in the chapter called The Unspoken Thing in The Electric Kool-Aid Acid Test by Tom Wolfe. It's also mentioned in a Beatles song, The Walrus. Remember the lyrics, "I, he, you, me, we, all, us, one..." In other words, we are all one.
This group consciousness was very important to the Pranksters. The idea was to put individual differences aside and work as a group...an attuned group. Those who weren't attuned were seen as rocking the boat, disrupting the trip. These were the people who were considered "off the bus." The ones who were attuned to the group consciousness, those who did their share of the work, and smile rather than pout, these were the ones who were "on the bus." Thus, the literal meaning of "You are either on the bus or off the bus" is "You are either attuned to the group consciousness or you are not attuned to the group consciousness." Well, isn't that groovy?
There are other metaphors that have the same meaning. Into the pudding is one. The one us audio engineers use is in the mix, as in, "You're either in the mix or you are not in the mix."